Research On The Neutral-point Voltage Balance Control For Back-to-back ANPC Three-level Converter

The active neutral point clamped three-level converter is improved on the basis of the diode clamped three-level converter.Compared with the diode-clamped topology,the switching device of each phase bridge arm based on the topology the loss is more balanced,the stand-alone capacity can be further improved,and the output performance is also more excellent,which is a recent research hotspot in the field of medium-voltage high-power.This article takes back-to-back ANPC three-level converter as the research object,and conducts in-depth analysis and research on the potential balance of its DC bus midpoint.This paper studies three carrier modulation methods(SPWM,DF-SPWM,PFSPWM,PFDF-SPWM)based on ANPC three-level converter,and secondly studies the traditional space vector modulation based on ANPC three-level converter(SVPWM),the virtual space vector modulation(VSVPWM)is proposed for the problems of traditional space vector modulation,and the theoretical analysis of its modulation strategy is verified by simulation,which provides theoretical support for the proposed hybrid VSVPWM method.In this paper,the working principle of the ANPC three-level rectifier is studied and analyzed.At the same time,the mathematical models of the ANPC three-level rectifier under the three-phase stationary coordinate system,the two-phase stationary coordinate system,and the synchronous rotating coordinate system are derived respectively.According to the mathematical model of its ANPC three-level PWM rectifier dq coordinate system,the double closed-loop control system based on direct current control strategy of ANPC three-level VSR is deeply studied,and the correctness of the theoretical analysis is verified by simulation.This paper analyzes the working principle of the ANPC three-level converter in detail,and makes an in-depth study on its commutation process.The midpoint potential fluctuation rate of the ANPC three-level converter is solved through quantitative analysis.The influence of each space vector on the midpoint potential of the DC bus of the converter is introduced in detail through space vector analysis.The principle of midpoint potential imbalance is analyzed.The effects of modulation degree,power factor,load type,and circuit parameters on the midpoint potential were further verified through simulation.This article analyzes the traditional SVPWM modulation principle in detail,and proposes a real-time neutral point balancing strategy for ANPC three-level inverters.That is,redefine the positive and negative small vector duty cycle coefficients,calculate the coefficient value required for the midpoint voltage offset to be 0 in the next sampling period according to the mathematical model of the midpoint current,and dynamically adjust the midpoint potential.With the background of the LCI drive system transformation,the problems of the traditional LCI drive system are introduced.A scheme for retrofitting the LCI traditional system based on the ANPC three-level inverter is proposed,and the midpoint potential control strategy proposed above is applied to it.Simulation The correctness of the strategy is verified.In this paper,based on the traditional space vector modulation method,the midpoint potential balance strategy has uncontrollable regions.Three midpoint potential balance strategies based on the virtual space vector modulation method are proposed.Among them,the hybrid VSVPWM redefines the virtual middle vector and defines the proportional parameters of each traditional vector in the virtual middle vector.By changing the value of the proportional parameter,the purpose of controlling the midpoint potential balance is achieved.Taking the back-to-back three-level converter system as the simulation platform,the control effect of the control system is simulated and analyzed through simulation,which proves the correctness of the strategy.